1. Field of the Invention
The present invention relates to welding of the inner surface of tubular members, and more particularly relates to an apparatus and method for laser welding the inner surface of a small diameter tube such as a nuclear heat exchanger heat transfer tube.
2. Background Information
In a conventional nuclear heat exchanger or steam generator, a heated and radioactive primary fluid flows through a plurality of U-shaped tubes, each of the tubes having a fluid inlet and a fluid outlet end. The inlet and outlet ends of the tubes are received through holes in a tube sheet disposed in the heat exchanger for supporting the tubes. The heat exchanger comprises an inlet plenum chamber below the tube sheet, which is in communication with the inlet ends of the tubes. The heat exchanger also includes an outlet plenum chamber below the tube sheet isolated from the inlet plenum chamber and in communication with the outlet ends of the tubes. During operation of the heat exchanger, a heated, radioactive primary fluid flows into the inlet plenum chamber and enters the inlet ends of the tubes. After flowing through the tubes, the primary fluid then flows through the outlet ends of the tubes and into the outlet plenum chamber where the primary fluid exits the heat exchanger. A non-radioactive secondary fluid having a temperature less than the primary fluid surrounds the exterior surfaces of the tubes above the tube sheet as the primary fluid flows through the tubes. As the heated primary fluid flows through the tubes, it gives up its heat to the secondary fluid surrounding the exterior surfaces of the tubes to produce steam that is used to generate electricity in a manner well known in the art.
Because the primary fluid is radioactive, the heat exchanger is designed such that the radioactive primary fluid flowing through the tubes does not commingle with and radioactively contaminate the non-radioactive secondary fluid surrounding the exterior surfaces of the tubes. The tubes are therefore designed to be leak-tight so that the radioactive primary fluid remains separated from the non-radioactive secondary fluid.
However, occasionally, the heat exchanger tubes may degrade and may not remain leak-tight. For example, the tube walls may be degraded by intergranular cracking caused by stress and corrosion occurring during operation of the heat exchanger. The tubes are therefore inspected to detect such stress corrosion cracking or degradation. In conventional heat exchangers, if stress corrosion cracking is detected at a particular location in the wall of the tube, the tube is then "sleeved" at that location. Such sleeving involves the insertion of a tubular metal sleeve into the tube which covers the degraded potion of the tube. The sleeves are typically affixed by expanding the sleeve into intimate engagement with the tube. However, the elastic properties of the metal sleeve may cause the sleeve to experience partial "spring back" after expansion. This phenomenon may cause a relatively small gap to exist at the sleeve-to-tube interface. Such a gap is undesirable because the gap defines a flow pattern between the sleeve and the tube that may allow the radioactive primary fluid to flow through any crack in the tube and commingle with the non-radioactive secondary fluid.
Welding techniques have been developed for fusing the sleeve to the tube by forming, for example, two spaced-apart weldments circumscribing the inner surface of the sleeve in order to seal any gaps between the sleeve and the heat exchanger tube. In particular, laser welding has been used to fuse such a sleeve to the tube.
A system and method for laser welding a tube is disclosed in U.S. Pat. No. 5,182,429 to Pirl et at. The system includes an elongated tubular housing having a rotatable distal potion connected to a non-rotatable proximal potion, a fiber-optic cable for conducting remotely generated laser light into the tubular housing, a beam deflection mechanism supported within the distal potion of the housing and a reflector for radially directing and focusing laser light received from the fiber-optic cable toward the inner wall of the sleeve. In order to weld around the inner wall of the sleeve, the system includes an electric motor within the proximal portion of the tubular housing to rotate the distal portion of the housing and the reflector supported therein.
Another system and method for laser welding a tube is disclosed in U.S. Pat. No. 5,371,767 to Pirl. This patent discloses a system for laser welding a sleeve having a relatively small inner diameter, e.g., 0.313 inch or less.
Additional laser welding systems and methods are disclosed in U.S. Pat. No. 5,006,268 to Griffaton, U.S. Pat. No. 5,066,846 to Pirl, U.S. Pat. No. 5,097,110 to Hamada et at. and U.S. Pat. No. 5,252,804 to Griffaton.
Although sleeving can restore the structural integrity of the heat exchanger tube, it has a number of disadvantages. The sleeve necessarily decreases the internal diameter of the tube passage adding increased pressure drop to the flow of coolant through the tube when the steam generator is placed in service. In addition, if the repair is located in the lower portion of a tube, such as near the tube sheet, subsequent repair of tube degradation above the location of the first sleeve may be prevented because another sleeve of the correct dimensions cannot be inserted past the already inserted sleeve. Furthermore, the welds made at both ends of the sleeve are usually recessed from the end of the sleeve because it is very difficult to accomplish a quality fillet weld on the end of the sleeve. Because these welds are recessed from the ends of the sleeve, a crevice remains between the sleeve and tube in the region between the end of the sleeve and the weld. Also, the area of the sleeve between the welds forms a crevice with the tube. The damage to the tube which necessitated the repair, such as a crack or a pinhole, may allow water to enter the crevice, making the crevice area susceptible to corrosion.
Attempts have been made to use a continuous, autogenous weld inside the tube, without the use of a sleeve, in order to repair damaged heat exchanger tubes. However, such sleeveless repairs can fail because the corrosion which led to the damage produces oxidized surfaces which result in flaws and voids when autogenous welding is used.
The above-noted laser welding systems and methods involve autogenous welding techniques wherein the weldment is formed by melting the metal of the sleeve and/or heat exchanger tube. The sleeve and/or tube thus provide the parent metal for the resultant weldments. While such autogenous weldments are suitable for many applications, a need exists for producing weldments from separately supplied filler metal.
The use of filler material in the welding process can introduce oxidizing and viscosity control agents into the weld zone, thereby preventing flaws and voids associated with autogenous welding. U.S. Pat. No. 5,359,172 to Kozak et al. discloses the use of filler metal for welding the internal surface of a heat exchanger tube.
U.S. Pat. No. 5,430,270 to Findlan et al. also discloses the use of filler metal to build up the internal surface of a heat exchanger tube, wherein a filler metal wire or sleeve is pre-positioned in the tube and a laser weld head is subsequently inserted in the tube. While the apparatus disclosed in U.S. Pat. No. 5,430,270 avoids the use of repair sleeves, it has several disadvantages including displacement and splattering of the weld coil during the welding operation, lack of sufficient penetration of the weld into the wall of the tube, and formation of a large heat affected zone which degrades the microstructure of the tube wall and leads to increased stress corrosion and intergranular cracking of the tube.
Each of the patents listed above is incorporated herein by reference.
It is apparent that improved mitigation techniques are needed to meet the future demands of nuclear power plants. Once the tube plugging margin has been exhausted and a large quantity of sleeves have been installed to permit continued operation of a heat exchanger, tube degradation eventually leads to a decision to replace the steam generator, de-rate the plant or decommission the facility. Alternative repair technology is needed in order to provide extended tube service at an economical cost.
The present invention has been developed in view of the foregoing and to remedy other deficiencies of the prior art.